Get postalcode on getDeliveryPriceByWeight() classes/Carrier.php

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Hello, 

I would like to get shipping postalcode on getDeliveryPriceByWeight() function in classes/Carrier.php ?

I tried 

$cart=new Cart();
$delivery_new = new Address((int)$cart->id_address_delivery);
$shipping_postcode =  $delivery_new->postcode;

and

$cart = new Cart();
$id_address = $cart->id_address_delivery;
if ($id_address) $shipping_postcode=self::getPostcodeByAddress($id_address);

but none of them works

Could you please help me?

Sorry for my english